The dominant or recessive nature of a mutation depends on the
ability of the mutant chloride channel monomers to polymerize with normal
channel monomers. Dominant mutations complex with normal monomers
producing defective channels. For some mutations one abnormal monomer is
sufficient to destroy the function of a tetramer complex (e.g. Pro480Leu).
For other mutations (e.g. Gly230Glu) it requires two abnomal monomers to
destroy the channel function of a tetramer. In either case, only a
minority of tetramers remain functional and myotonia results.
Recessive mutations do not complex with normal monomers. Normal
monomers are then free to complex with other normal monomers. This
produces enough functional tetramers in heterozygotes (50% of the usual
amount) to preserve normal membrane excitablity and myotonia does not
occur.

What are the properties of the mutations in the sodium channel gene (SCN4A)
that determine whether a syndrome presents with myotonia, paramyotonia, or weakness?

Many mutations produce abnormal inactivation of the sodium channel.
This results in increased sodium conductance and membrane depolarization.
Mild depolarization is associated with increased membrane excitability and
myotonia. Strong depolarization produces membrane inexcitability and
weakness. Some mutations only reduce inactivation at low temperatures
producing paramyotonic disorders (myotonia or weakness worse in the cold).
Mutations in the inactivation gate (amino acid 1306) produce different
degrees of disease severity depending on the size and charge of the side
chain of the new amino acid. Alanine, with a short side chain produces
mild
myotonia fluctuans. Valine, with an intermediate side chain, produces
paramyotonia congenita. Glutamic acid, with a long side chain and a
negative charge, results in
myotonia
permanens.